1. Field of Invention
The present invention relates to a polarization independent tunable acousto-optical filter and the corresponding method. More specifically. it relates to a polarization independent tunable acousto-optical filter that can be applied to dense wavelength-division multiplexing (DWDM) systems and the method of the same.
2. Related Art
When an acoustic wave passes through a medium, the structure in the medium will generate a periodic elastic deformation so that its refraction index also has a periodic variation. This is equivalent to a moving phase grating that can diffract incident light. This phenomenon is called the acousto-optical effect.
An acousto-optical tunable filter (AOTF) is a device utilizing the acousto-optical effect. When input light contains many distinctive channels at different wavelengths, these channels can be individually extracted through diffraction by changing the modulation frequency of an external RF generator. Since the central wavelength of the interacting light can be adjusted by the external RF frequency and existence of multiple interacting bands is allowed, therefore the tunable acousto-optical filter can be useful when applied to dense wavelength-division multiplexing (DWDM) systems.
As a key element in a DWDM system, the tunable acousto-optical filter has to have such properties as high resolution, high side-lobe suppression ratio, and polarization independence. The high resolution condition means that one can precisely define a specific frequency band for light to be diffracted. The high side-lobe suppression means that only a specific band is filtered, leaving light with other frequencies intact. The polarization independence means that the filtering effect is independent of the polarization of the input light. The prior art, as disclosed in the U.S. Pat. Nos. 5,329,397, 535,945,1, 5,410,371, 5,946,128, and 5,909,304 cannot simultaneously satisfy the above three requirements.
Tunable acousto-optical filters can be categorized as collinear and non-collinear types. The non-collinear type refers to the propagation of optical waves and acoustic waves along different axes. The U.S. Pat. No. 6,016,216 proposed a non-collinear polarization independent tunable acousto-optical filter that can satisfy the above-mentioned three requirements. However, the polarization beam splitter (PBS) used in the method disclosed therein is expensive and the propagation of the light is not along a single axis. Therefore, it has a big volume and a low extinction ratio. Since the separating distance between beams is too big, two transducers are needed and the necessary microwave energy is severely high.
In view of the foregoing, how to provide a tunable acousto-optical filter that can simultaneously satisfy the requirements of high resolution, high side-lobe suppression, polarization independence, minimized volume, and low cost are important subjects to be studied.
Therefore, the present invention provides a polarization independent tunable acousto-optical filter and the method of the same. The filter transmits an input light beam into a first output light beam unaffected by acoustic waves and diffracts a second output light beam being affected by acoustic waves.
The disclosed invention uses a first polarization beam displacer/combiner to separate the input light beam into a first polarized light beam and a second polarized light beam orthogonal to each other. A first polarization rotator rotates the polarization of the first polarized light beam by 90 degrees. An acousto-optical device generates the acousto-optical effect to rotate the polarization of the light of a particular wavelength by 90 degrees. A second polarization beam displacer/combiner separates a third polarized light beam from the first polarized light beam and a fourth polarized light beam from the second polarized light beam. A second polarization rotator rotates the polarizations of the second and the fourth polarized light beams by 90 degrees. A third polarization rotator rotates the polarizations of the third and the fourth polarized light beams by 90 degrees. A third polarization beam displacer/combiner combines the first polarized light beam and the second polarized light beam into a first output light beam that is unaffected by acoustic waves and combines the third polarized light beam and the fourth polarized light beam into a second output light beam that has a specific wavelength diffracted by the acoustic wave.
It is thus an object of the present invention to provide a tunable acousto-optical filter and the corresponding method that satisfy such requirements as high resolution, high side-lobe suppression, and polarization independence, and can minimize its volume and lower its cost.